In applications for identification of persons and things, optical bar coding technology is almost universally employed. Generation of the bar code is very inexpensive but limited. One problem associated with bar codes and bar code readers is that the bar codes must be precisely aligned with the bar code reader in order to be read. In situations requiring generalized scanning, several scanning laser beams are used, each at a different angle of incidence to the object being read. Another problem with bar codes is that the bar codes may become unreadable as a result of damage due to, for example, exposure to moisture, or wear and tear from use. Radio frequency identification (“RFID”) tags address some of the shortcomings of bar codes and have been proposed as a replacement for optical bar codes in at least some applications. RFID tags used in bar code applications are sometimes referred to as electronic bar codes.
Remotely powered electronic devices and related systems for powering up and receiving stored information from such devices are well known. For example, U.S. Pat. No. 4,818,855 issued to Mongeon et al., titled, Identification System, discloses a remotely powered identification device which derives power from a remote source via one of an electric field or a magnetic field and which transmits stored information back to the source via the other of the electric field or magnetic field. Remotely powered identification devices of this type are commonly referred to as RFID tags. A power source with a data collection function is known as a tag reader. A power source capable of sending data to a tag is known as a tag writer. A power source capable of bi-directional communication is known as a tag reader/writer.
An ongoing objective in the development of RFID tags and associated readers and/or writers of the general type described above has been to minimize cost and size, and to improve efficiency of operation. The simplest and least expensive RFID systems employ unidirectional communication, allowing data transfer from tag to reader only. These are commonly known as read-only systems. In read-only systems, eliminating the need for a data receiver on the tag minimizes tag cost. Typically, these tags transmit information continuously as long as they receive adequate power from the source, wherein lies the primary system limitation. The reader's receiver is capable of reliably detecting data from only one tag at a time. If multiple tags are present within the reader's field, they will simultaneously transmit and create mutual interference at the reader's receiver, preventing the data from any one tag from being recovered successfully. This mutual interference condition is commonly referred to as a data collision. The terms anti-collision and collision mitigation are used to describe methods employed to prevent or minimize the impact of such data collisions at the reader.
Prior RFID systems have used the Aloha protocol for anti-collision. The Aloha protocol requires substantial bi-directional communication between the reader and the tags. The Aloha protocol sorts through a population of RFID tags and assigns each tag a unique node address. This node address is subsequently used to provide collision free communication between the tags and the reader. The reader sends out a request command to all tags in the field. The tags react to the request command by selecting a random number. This random number defines the tag's channel identification or slot number. The reader polls the tags in the field looking for a response. The reader starts by polling for slot number 0. All tags that have chosen a random number of 0 respond. If exactly one tag responds, then the reader assigns a unique node address to that tag. If more than one tag responds, a collision will occur. The reader will ignore this indecipherable response. If no tags respond, the reader moves onto the next slot. This process continues by polling for slot number 1. Again, if a single response occurs, the tag is assigned a unique node address; otherwise, the polling sequence proceeds with the reader polling for the next slot number. Upon reaching the last slot, the reader can start over by requesting tags that have not been assigned a node address to select a new random number. The entire polling process is repeated until all tags in the field have been assigned unique node addresses. At this point, the reader can select an individual tag for subsequent communication by specifying its unique node address, providing a collision-free communication channel.
A problem with the Aloha network scheme is that if there are many devices, or potentially many devices in the field (i.e. in communications range, capable of responding) then there must be many available slots or many collisions will occur. Having many available slots slows down replies. If the magnitude of the number of devices in a field is unknown, then the optimum number of slots needed is also unknown, and the system chooses a large number of slots, in an attempt to choose a number larger than three times the number of devices in the field. If the size is grossly overestimated, the system will slow down significantly because the reply time equals the number of slots multiplied by the time period required for one reply. Therefore a need exists to increase throughput well above the theoretical time that would be allowed by a standard slotted Aloha network scheme.